Apparatus for vibration testing



Aug. 3, 1943. J. A. HUTCHFSON APPARATUS FOR VIBRATION TESTING Filed Aug.2, 1940 Beat fiequency Oscillator INVENTOR 'John A Hutches on.

ATTORN' 4 73. MEASURWG & TESTINQQ Search Room APPARATUS FOR VIBRATIONTESTING John A. Hutcheson, Catonsville, Md., assignor to WestinghouseElectric & Manufacturing Company, East Pittsburgh, Pa., a corporation ofPennsylvania.

Application August 2, 1940, Serial No. 349,403

(Cl. iii-51) 1 Claim.

This invention relates to apparatus for testing the physical propertiesof materials, and more particularly to testing of fatigue undervibratory tension.

The primary object of this invention is the provision of efficient yetsimple coordination of means for vibrating materials or mechanical systems over a wide range of frequencies, in order to produce thereinartificially, within a relatively short time, mechanical stresses towhich such materials or systems may be exposed under operatingconditions in their intended fields.

Fatigue tests of different materials or mechanical assemblies composedthereof are difficult to perform, requiring considerable time andtroublesome apparatus. .Heretofore, such tests were frequently performedby machines which produced the required stresses in a specimen by purelymechanical power transmissions at an arbitrary rate, the frequencythereof being limited to a narrow range due to the mechanical inertia ofthe component parts. Especially in testing vibratory tensions, thenatural periodicity of vi- I bration of the specimen under test couldnot be accomplished within the narrow frequency range of such apparatus.

A particular feature of the present invention resides in the widefrequency range of the apparatus which is progressively adjustablewhereby the natural vibration frequency of the specimen under test mayeasily be determined, and furthermore vibration at this desiredfrequency automatically maintained at the proper amplitude.

In conducting fatigue tests which usually must run for a considerableperiod of time, it is extremely desirable to be able to inspect thespecimen under tests at the critical period just before it physicalstructure is disrupted at a time when changes take place therein whichultimately will cause its failure.

It has been found that a test specimen when vibrated by a motor suppliedfrom a constantvoltage power source quickly attains a steadystateamplitude of vibration which continues until the vibrations begin tocause mechanical failure in the specimen, whereupon the vibrationamplitude undergoes a change. A salient {eature of the present inventionis that means are provided for automatically stopping the operation ofthe apparatus at a desired and predetermined phase in the operatingcycle when changes in the amplitude of vibration of the object undertest is indicative of ensuing failure. In this manner, the objectsubjected to test can be observed in order to determine the physicalchange prior to such ultimate physical destruction which oftenobliterates the marks of the incipient cause producing it.

The apparatus in accordance with the present invention is a modificationof that; described in my copending application Serial No. 249,259, filedJanuary 4, 1939, for-Vibration fatigue apparatus, and assigned to theWestinghouse Electric 8: Manufacturing Company, and compriseselectromechanical means for producing vibratory tension in materials tobe tested together with means for maintaining the tension at a frequencyrate corresponding to the natural frequencyof the material, and meanswhich respond to any deviation from an-initial amplitude of vibrationand operates automatically to stop the application of further vibratorytension.

Further objects and advantages will be apparent from the followingdescription of the invention, particularly pointed out in the appendedclaim, and taken in connection with theaccompanying drawing, which "is aschematic. circuit diagram of the apparatus.

Referring to the drawing, an alternating-current supply line i and 1',controlled by a circuit breaker 2 having winding 3 and contacts 4 and 4,is connected by means of conductors 5 and 5' to the various componentsof the apparatus. The circuit breaker 2 has its winding 3 connected insuch manner that upon completion of the circuitthrough the start button6 it is energized from the power supply line and remains in theenergized position until the circuit is broken by either the stop button1 or the contacts 8 of the relay 9. The power supply'line conductors 5and 5' are connected to the primary winding ll! of the power transformerH and to the energizing circuit of a beat frequency oscillatordiagrammatically represented by a conventional square. The powertransformer l I has a number being connected to the high potentialsecondary winding ll of the power transformer H, which is center tappedto ground of the system. A second rectifier tube 18 in full-waveoperation has its filament 19 connected to another secondary secondarywinding 22 is connected by means of conductor 23 in series with filterreactors 24 and 24' through one terminal of the voltage divider resistor25, the other terminal of which is connected to ground. Completing thecircuit to the rectifier tube I8, the filament l9 thereof is alsogrounded. Condensers 26 and 26 connected to of the tube utilized, Thepotential across the resistor 30 will be increased in the positivedirection from the grounded terminal thereof, whereas across theresistor 25 this condition is reversed and the potential will be morenegative with respect to ground.

The main components of the apparatus comprise a beat frequencyoscillator, formerly mentioned which is' of the type that can be variedin frequency output in the audio frequency range from approximately 10to 10,000 cycles, a power amplifier, which can be excited either fromthe oscillator or from another source, and an amplitude responsivecircuit which actuates the relay 9.

The power amplifier consists of a pair of power amplifier tubes 3! and3| connected in pushpull operation. The inputcircuit betweeninterconnected and grounded cathodes 32 and 32' and grids 33 and 33,respectively, comprises the divided portions of winding .34 of the inputtransformer 35, the center tap of the winding 34 being connected byconductor 36 to a suitable tap 31 of the voltage divider 25'. The bypasscondenser tubes, having also a screen electrode, which are connectedtogether and also to the conductor 42. Being thereby of a potentialsupplied to the anodes, the screen electrodes are bypassed by condenser43. The output transformer is shown having two secondary windings 44 and44'. One of these windings 44 is connected to the armature winding 45'of the vibration motor 48 by means of conductors 41 and 41', Thesecondary winding 44' is not connected anywher in the system, and is forthe purpose to supply energy to a different type of vibration motor thanis shown here.

Referring now'tothe input circuit of the amplifier 3|, 3|, the inputtransformer has a. divided primary winding 73, one terminal of whichthrough conductor 79 connects to two moving arms of a four-pole,double-throw switch 80. Another moving arm of the switch connectsthrough conductor 8| to the tap 82 of the winding 18, whereas the otherterminal thereof con, nects through conductor 83 to the remaining movingarm of the switch 86. Two contacts of the switch 80 in one position ofthe arm thereof connect through conductors 84 and 84' to a potentiometer85, the resistance of which is in parallel with another winding 86 ofthe armature 81 of the vibration motor 46. The armature 81 is providedwith spring supports lill which permit armature 45 to vibrate and with asuitable mechanical coupling means shown here in the form of a bar 81'for transmitting the vibratory motion to various specimens under fatiguetests. The other two contacts in the reverse position of the switchconnect through conductors 88 and 88' to the terminals of the beatfrequency oscillator.

In order to discontinue the vibrationof the test piece when mechanicalfailure is about to occur therein, the following circuit arrangement isused: Extensions of the leads 84, 84 are connected to the primarywinding 6!] of a transformer having its secondary windings 6| con nectedin the same push-pull arrangement to the anodes of two thermionicrectifiers 62, 63. In series with this pair of rectifiers is connected2. potentiometer 64 shunted by a smoothing capacitor 64a in accordancewith conventional practice. The negative end of the resistor 64 isconnected to a variable tap 65 on the potentiometer 30, and the movabletap on the potentiometer 64 is connected to the grid 66 of an amplifiertube 6'! of suitable type. The cathode of the amplifier 6'! is connectedby a variable tap -68 to the potentiometer 30 and also through threeserially-connected resistors ll, 72 and 73 to the anode of the tube 6'1,The common junction of the resistors 72 and 73 is connected to avariable tap 14 on the potentiometer 30, and the tap T4 is alsoconnected through the energizing winding 75 of a relay, the other end ofsaid energizing winding being connected to the comon terminal of theresistors H and '12. A pair of separable contacts, controlled by thewinding 15, is con nected in series with a source of voltage l5 and thecontrol winding 75 of the relay 9 which has previously been described ascontrolling the main circuit breaker contacts 4, 4.

It will be noted that tube 61 and the three resistors "ll, '12 and [3form a bridge connection 'in which the winding 15 is connected acrossone diagonal, while voltage derived from the potentiometer 30 isconnected across the other diagonal. The equivalent resistance of thetube 67 may be varied byvarying the potential of its grid 65 relative toits cathode, and this latter may be accomplished by varying the settingof the variable tap on the resistor 54. The current through the windingI5, which is connected across a diagonal of the above-mentioned bridge,may accord-- ingly be made zero or adjusted to any other desired valueby varying the variable tap on the resistor 64.

The field winding 98 of the vibration motor 43 is paralleled by a fielddischarge resistor 9| and connects by means of switch 92 and seriesresistor 93 to a suitable. direct-current potential source. A pilotlight 94 is provided to indicate energizetion of the winding 90.

Describing the operation of the system herein disclosed, let it beassumed that the power switch 2 and the switch 92 are in closedposition, whereby the necessary alternating and direct-current supplypotentials are connected to the system. The amount of power expended inthe field wind- -ing can be controlled by the resistor 93, and

the presence of field voltage across this winding is indicated by thepilot light 94. The resistor BI,

73.. WEA

SUREHG d vibration motor 46. The application of alternattug-currentenergizes also the audio frequency or beat frequency oscillator. I

The output of the bias rectifier I8 is filtered by reactors 24 and 24',capacitors 28 and 26', and

is impressed across voltage divider resistor 25; This provides properbias for the operation of the vacuum tubes in the amplifier andamplitude responsive control tubes. The output of the rectifier tubes Iand I5 isfiltered by reactors 28 and 28', capacitors 29 and 2S, and isimpressed across the voltage divider 3D. This supplies anode potentialto the tubes aforementioned.

As shown in the drawing, when switch 88 is closed downward the beatfrequency oscillator output is directly connected across the primarywinding 18 of the input transformer 35. In this particular position,the'amplifier is excited by the oscillator output voltage, and theamplifier output will energize the winding 85 of the vibration motorarmature 87. The test specimen will vibrate at the frequency determinedby the setting of the oscillator which may be varied between a widerange, as previously stated. The vibration motor will operate as long asswitch 2 is closed, and thereby power will be delivered to the system.The opening of this switch can be manually effected by pressing button Ior by energizing the winding 16 of the relay 9, which then opens thecontact 8.

As the test specimen vibrates, a voltage will be generated in thewinding 86, and. this voltage will be impressed through the primarywinding 60 on the rectifier tubes 62, 83 and thus produce a di- SearchRoom a inasmuch as it is derived from the movement of the armature. Thewinding 86 is so connected with respect to phase relation to the inputcircuit of the amplifier that the voltage derived therefrom isregenerative; that is, in aiding phase relation with the voltage in theoutput circuit of the amplifier. The latter will operate now with v thexcitation from the winding 86, being entirely self-excited at thenatural frequency of vibration of the specimen under test.

For a given setting of the potentiometer 85, the tubes 3!, 3i willquickly bring the vibrations of the test specimen'to a steady-state ofamplitude. The potential drop through the resistor 64 will varyinaccordance with variations in this amplitude, but the variable tap onresistor 64 may be set so as to give the current flowing through winding75 such a low value that it is incapable of opening the contacts 8 ofthe relay 9. As previously stated, it has been found that the amplitudeof vibration of the test specimen will remain substantially constantuntil mechanical failure of the latter begins to occur. When mechanicalfailure starts, the amplitude of vibration of the test specimen willundergo a change,- thereby varying the potential drop through theresistor 64 and causing such a change in the current through winding 15as to cause current rect-current potential drop through the resistor 54.The variable tap on the resistor 64 may then be set to so adjust theequivalent resistance through the tub 61 that current through thewinding 75 is insufiicient to move the contacts of the relay associatedwith winding 15 from their normally open position. In consequence ofthis adjustment, the winding '15 of the relay 9 will be deenergized andthe contact 8 of the latter remain closed, so that the contacts 4, 4" ofthe from the voltage source 15' to open the contacts 8 and thereby openthe contacts 4, 4' of the main power circuit breaker and discontinue thevibration of the test specimen.

In most cases it will be found that maximum sensitivity of the relay 9to variations in the amplitude of vibration of the test specimen will beattained if the voltage drop through resistor 84 is opposite in polarityto that between taps 65 and 68 of potentiometer so as is indicated bythe pres ent drawing. However, opposition of these polarities is notabsolutely essential to cperativeness of my arrangement.

main power circuit breaker likewise remain closed.

When testing the fatigue characteristics of materials or mechanicalsystems, the oscillator output is slowly varied in frequency from aminimum to a maximum, or vice versa, and the test mae'rial put undervibratory tension over a range iii-frequency. At a particular frequency,it'will be observed. usually by an audible indication, that the beatfrequency and the natural period of vibration of the material under testcoincide, in which case, the natural frequency of vibration should bemaintained for the test, and the switch 80 may then be closed into itsupper position, as

shown on the diagram. In this position, the wind- 4 ing 86 iseifectively connected between tap 82 and conductor '19 to a portion ofthe primary winding of input transformer 35. The movement of thearmature 81 in the magnetic field produced by the winding 98 induces avoltage in the winding 85, which is now utilized to excite the input ofthe amplifier. Ihe magnitude of voltage necessary for energizing theinput of the amplifier may be determined by, varying the potentiometerThe voltage so derived is alternating in character having a frequencycorresponding to the natural frequency of vibration of the specimen,

Since structural details of the vibration motor 46 form nopart of thepresent invention, they will not be described, but reference is made tomy copending'application previously mentioned for information concerningthem.

Certain subject matter described herein is claimed in my above-mentionedcopending application.

I claim as my invention:

In an apparatus for testing the physical properties of materials, areciprocating motor adapted to vibrate said materials and comprising apair of relatively movable windings, one physically stationary relativeto the support for said material and the other mechanically connected tovibrate with said material, means for energizing said one winding withdirect current, an electron I tube having an output circuit and acontrol electrode circuit, said output circuit being connected toenergize said other winding, means for coupling said control electrodecircuit to said output circuit with such a polarity as to cause saidtube to generate electrical oscillations of the natural frequency ofsaid material, a circuit breaker hav-- ing control circuit, a rectifierconnected to supply energy from said control electrode circuit to thecontrol circuit of said circuit breaker, a current source bucking theenergy supplied by said rectifier in said control circuit, said circuitbreaker being arranged to deenergize the power supply circuit for saidelectron tube in response to a predetermined change of current throughsaid rectifier.

JOHN A. HUTCHESON.

